1. Field of the Invention
The present invention relates to one-time programmable cells for use in integrated circuits, and more particularly, anti-fuse cells.
2. Discussion of the Related Art
One-time programmable cells are cells which can be programmed from one state to another on one occasion, after which further modification is not possible, and the result is non-volatile.
Included in this category are fuses, which can be divided into fuses and anti-fuses. In the case of fuses, pre-existing connections are broken permanently during the programming whereas for anti-fuses, permanent connections are made between previously unconnected nodes.
A first type of fuse consists of a polysilicon track with a narrowed section and terminals for the application of a current. The track presents a resistance of a few tens of ohms between the terminals. When a high current is passed through the fuse, localized heating of the narrowed section occurs and fuses the track, making the resistance greater than one megohm. During the fusing process, the surrounding layers are liable to be damaged significantly, and the break is sometimes unreliable because residual filaments of polysilicon can remake the connection.
A second type of fuse consists of a metal track which provides a resistance of less than one ohm between the terminals. To program the fuse, laser radiation is used to fuse a portion of the track whereby a break occurs, creating a resistance greater than one megohm.
A first type of anti-fuse consists of an insulated gate MOS transistor. The terminals of the anti-fuse are formed by the gate electrode on one hand and the source and drain connected in common on the other, between which is presented a resistance greater than one megohm for the un-programmed fuse. The application of a high voltage between the gate and the substrate causes the rupture of the gate oxide creating a resistance in the order of hundreds of ohms between the gate electrode and the common source/drain electrodes.
A second type of anti-fuse consists of a MOS transistor connected in the off-state, its source and drain electrodes forming the terminals of the anti-fuse cell. When a high voltage is applied between the source and the drain of the MOS transistor, inducing the passage of a high current, permanent defects bridging the source-channel and drain-channel junctions are created. This results in a permanent connection of a few kilo-ohms between said source and drain
So, generally, the electrically programmable fuses and anti-fuses, such as the above, have the disadvantage of requiring a high voltage for their programming, and hence a specific voltage supply source. Moreover, this voltage is significantly above the normal operating voltages of an integrated circuit technology, which requires the use of special/non-standard devices and process options.
The non-electrically programmable fuses present the disadvantage that programming is only possible while the integrated circuits are still in wafer form at the manufacturing site. Thus programming of product after packaging, for example by the end customer, is impossible. This programming is also time-consuming.
Also, generally known anti-fuses present a relatively high resistance when closed, excluding them from use in many applications, and often imposing an additional discriminator circuit to detect whether or not the link is closed.
Furthermore, generally known fuses and anti-fuses present a disadvantage in that the effect of programming cannot be verified without actually programming the cell. The decision to program a cell, often based on a prior measurement and a calculation, is open to error which results in the entire integrated circuit being rejected.